US3491192A - Heat insulating lining for electric furnaces - Google Patents

Description

Jan. 20, 1970 P. H. H. BISHOP E AL 3,491,192

HEAT INSULATING LINING FOR ELECTRIC FURNACES Filed May 24, 1968 3 Sheets-Sheet 1 FIG. I

HEAT INSULATING LINING FOR ELECTRIC FURNACES Filed May 24, 1968 I 1970 P. H. H. BISHOP ET AL 5 Sheets-Sheet 2 I I l/ 1 I I l I 3,491,192 HEAT INSULATING FOR mums ELECTRIC FURNACES Filed May 24, 1968 Jim. 1970 P. H. I-t BISHOP ET AL 5 Sheets-Sheei 3 FIG. 3,

United States Patent 3,491,192 HEAT INSULATING LINING FOR ELECTRIC FURNACES Peter Harold Howard Bishop, Farnham, and Kenneth Francis Rogers, Farnborough, England, assignors to National Research Development Corporation, London, England Filed May 24, 1968, Ser. No. 731,784 Claims priority, application Great Britain, May 24, 1967, 24,07 8/ 67 Int. Cl. H05b 3/62 U.S. C]. 13-20 6 Claims ABSTRACT OF THE DISCLOSURE A furnace is made up of an outer case 1 of electrically insulating hardboard and a lining comprising a mass of spirally coiled rolls 2 of carbon fibres packed in to define and thermally insulate an inner charge-receiving space and to aiford support for a charge which can be placed on a carbon hearth 4. The axes of the rolls are substantially normal to the charge-receiving-space boundaries which they define. The rolls are made of carbonised cotton wool; they may be pyrolisedat least in part-in situ, secured to the walls of the case by ordinary adhesive.

Tubular electric resistance-heating elements 6 are mounted in insulating tubes 11.

The walls of the case are of sandwich form, the middle layer 9 having an opening 9a forming a manifold receiving inert gas from a pipe 5 and communicating with the interior of the furnace through the pores of the whole mass of carbon fibre rolls. A pipe 12 for discharge of gas from the furnace passes through one of the heaters 6.

In one conventional construction of large furnace for operating at somewhat high temperatures-Le. 1500 C. and abovean outer casing (usually of steel) encloses an inner furnace lining of carbon blocks supported from the outer casing. The lining is separated from the outer casing by a wide gap which is packedbetween the supports-with thermal insulation in the form of carbon granules or felt and usually ordinary lampblack. Such a furnace is used of course for heating a charge in a vacuum or a non-oxidising atmosphere. Both in manufacture and during operation the dirty nature of the lampblack and the fact that it is granular makes it very difficult to handle and causes soiling of the surroundings whenever the insulating lining has to be opened up for inspection or repair. The lampblack continuously tends to settle and needs to be topped up from time to time. The supports extending across the heat-insulating gap from the outer casing to the inner lining conduct some waste heat across the gap and lower the efliciency of the furnace.

With a view to overcoming some of the aforesaid disadvantages the present invention is concerned with a cocoon construction of heat-insulating fibres which replaces the lampblack, can partly or wholly eliminate the supporting rods passing across the heat-insulating space and hence can reduce heating of the casing and loss of efliciency by conduction through the supports, and which can wholly eliminate the carbon slab lining except perhaps for a heart-h on which the charge can lie. Thus according to the invention a heat-insulating packing in a furnace is a mass of spirally coiled rolls of heat-insulating fibres tightly packed in to define and thermally insulate an inner charge-receiving space, and also affording support for a charge in the furnace. Preferably the rolls are stacked with their axes normal to the part of the outer casing which supports them and hence normal to the surfaces of the charge-receiving space which they define. Such spirally coiled rolls of fibrous material can serve as structural elements supporting substantial loads as set forth in our prior British patent specification No. 921,061, and thus this fibre packing and lining serves both as heat insulation and support for the weight of the charge. In particular the packing is of carbon fibres, these being good thermal insulators, non-friable andunlike carbon in the form of lampblack powder-reasonably clean to the touch. A cheap and useful material is cotton waste which has been carbonised by heating to a carbonising temperature in a non-oxidising atmosphere. Textile waste of other natural or synthetic fibres may be used.

One particular construction according to the invention is illustrated by the accompanying drawings of which:

FIG. 1 is a front view of a furnace with the door open FIG. 2 is a section on IIII of FIG. 1 and FIG. 3 is a section on III-III of FIG. 2.

An outer casing 1 made up of bottom, top and side plates and a blackplate, but with the front left open for inserting the charge into the furnace, is lined with a filling of closely packed spirally coiled rolls 2 of carbon fibre, some rolls being stacked on the bottom of or depending from the top of the casing with their axes 2a substantially vertical and others being stacked against the sides and back of the casing with their axes 2b substantially horizontal. The inner ends 212 of these rolls 2 form the faces of and so define an inner charge-receiving space, each face thus being substantially normal to the axes 2a or 2b of the rolls forming it. The rolls are chosen of sufficient length to provide the necessary heat insulation" between the charge and the outer casing; the drop in temperature along each roll will be sufficient for the temperature of their outer ends 2d to be so much lower than the temperature within the inner space that the rolls may be held securely by attaching these outer ends 2d to the inner surfaces of the casing walls by ordinary adhesive.

Each roll 2 may be made by taking a commercially available roll of cotton wool, or rolling a sheet of any fibrous carbonisable material, and heating it to a carbonising temperature in an inert or at least non-oxidising atmosphere, taking it up to what will be at least the working temperature of the furnace.

The walls of the casing 1 may be of metal, as is conventional, but since they will be at low temperature they may be hard boards of an electrical insulating material. They are built up as described later.

The front access opening can be closed by the hinged door 3 (FIG. 2) which is built up of a composite plate carrying a set of the fibre rolls 2, similarly to the walls 1. The slightly soft and yielding nature of the fibrous rolls greatly reduces risk of jamming of the door and consequent damage due to a slightly badly fitting door.

Resting on the rolls 2 standing up from the bottom plate of the casing 1 are a number of carbon blocks or tiles 4 forming a charge-receiving hearth; the weight of the hearth, loaded by a charge, is taken by axial compressive loading of the spiral rolls 2. If the nature of any charge in the furnace be such that by its weight it imposes some load on the sides and back of the furnace this load will also be taken by axial compression of rolls 2 One wall of casing 1 has an opening for pipe 5 for the admission of the gas-in particular an inert gas, or a gaseous mixture which does not include oxygenwhich may be required as the atmosphere in the furnace for any particular process or for the admission of purge gases before changing from one atmosphere to another.

Each wall of the casing 1 is made up of inner and outer plates 7 and 8 between which is sandwiched a plate 9 pierced by an opening 9a forming a manifold. The pipe 5 opens into the manifold in one of the plates 9. Also in communication with each manifold is a set of holes 9b in plates 7 and 9 opening onto the rear end faces 2d of the rolls so that gas may pass between the manifold and the rear of each mass of fibre rolls. The manifold 9a in each side of the casing 1 communicates with the manifold in a neighbouring side wall through a short pipeelbow 10 passing through the middle plates 9 of each side; similarly one side wall communicates with the interior of the top wall and one side also communicates with the interior of the bottom wall. Thus pipe 5 is in communication with all the manifolds 9 and thence with the rear of all the masses of fibre rolls on the casing 1.

Pipe 5 can be used to fill the furnace with the desired atmosphere before use. When, after use, the furnace is opened up by opening the door 3, the temperature inside may still be high enough for incoming atmospheric air to ignite some partof the carbon fibres; accordingly pipe 5 is again used to introduce pressurised cooling protective gas starting before the furnace is opened, the gas being inert or at least incapable of supporting combustion. This gas can pass through the pores of the whole fibrous mass into the interior space of the furnace and oppose entry of air into the fibrous mass. When the furnace is being initially filled with gas the furnace will not be so completely sealed as to prevent leakage of the air which is being driven out, or there may be an escape through a further pipe 12 provided as described hereinafter. To guard against risk of displacement, wear or damage of one roll putting one of the holes 9b into direct communication with the interior space and thereby draining off all the gas from the other holes 9b, each of these holes communicates with its associated manifold 9a through a narrow slit 9c in the plate 9, the drop of pressure along this slit being comparable with that in the pores of the fibrous mass.

Preferably, as shown, the door 3 is constructed similarly to the walls of the casing 1 so that inert gas can be passed through the fibrous mass thereon. To permit movement of the door between the open and the closed position the pipe-elbow 10a between the top of the casing and the door is flexible. Gas passing through the pores in the bottom plate of the casing 1 will enter the inner space through the gaps between the individual blocks of the hearth 4.

The furnace is electrically heated by tubular nonmetallic resistance-heating elements, two of which are shown in external view at 6. Since the carbon-fibre rolls are usually electrically conductive the elements must be insulated from them. Accordingly, two bushes 11 of electrical insulating material pass through the mass of fibrous filling on the backplate of the furnace. The elements 6 pass through entry holes in the backplate, through the bushes 11, and extend to the front of the charge-receiving space as rear-supported cantilevers. Preferably these elements are of high resistance, taking comparatively low current at a voltage of the order of the voltage of supply mains.

A further pipe communicating directly with the interior of the charge-receiving space may be desired. This may for example be necessary for continuous escape of gases which may be driven off by some heating processes, and such pipe may be required to be kept hot by proximity to the source of heat; such a pipe is shown at 12 connected to the interior of one of the tubular heaters 6 and communicating with the furnace interior through openings 6a. A seal 13 is provided between the elements 6 and the backplate around the pipe 12 to reduce leakage of gas by any path other than through pipe 12.

In making the furnace the fibre rolls 2 may be assembled therein in the uncarbonised or in an only partially finished state and then be carbonised or matured by pyrolysis in situ.

We claim: I

1. A furnace made up of an outer case, heating means carried by said case and a lining within said case, said lining comprising a mass of spirally coiled rolls of heatinsulating fibres packed into said case to define and thermally insulate an inner charge-receiving space and to afford support for a charge.

2. A furnace according to claim 1 wherein said lining comprises rolls spirally coiled around axes substantially normal to the charge-receiving-space boundaries which they define.

3. A furnace according to claim 2 wherein said rolls are of carbon material.

4. A furnace according to claim 3 wherein said rolls are of carbonised cotton-wool.

5. A furnace according to claim 3 comprising gas connections to the rear of said rolls for the passage of nonreacting gas between said connection and the chargereceiving space by way of the pores in the whole mass of rolls.

6. A furnace according to claim 1 wherein said heating means includes a high-resistance electric heating element.

References Cited UNITED STATES PATENTS 1,517,820 12/1924 Bong -95 2,657,247 10/ 1953 Bretschneider 13-20 X 3,213,177 10/1965 Diefendorf 1320 X BERNARD A. GILHEANY, Primary Examiner H. B. GILSON, Assistant Examiner US. Cl. X.R. 13-31

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